When I get messages about ‘shedding,’ my almost-40-year-old brain immediately goes to the hair that I pull out of my shower drain, which seemingly gets worse as I hurtle towards perimenopause. But alas, most of the questions are actually about this idea of ‘vaccine shedding’ - the notion that vaccinated people are somehow dangerous to be around.

Now, this isn’t new. You can find fact-checks and responses to this question from a few years back (remember when that Miami school banned vaccinated teachers? Wild times). But the myth keeps circulating, morphing into new forms, finding new audiences.

As immunologists and public health experts, we often get asked about this topic—especially in the wake of misleading information surrounding COVID-19 vaccines. The term sounds alarming, but let’s break down the science and explore what it really means, when it happens, and why it’s not something most people need to worry about.

So let’s start with the basics: what vaccine shedding actually is, why certain vaccines can cause it (while others absolutely cannot), and why you can safely hug your vaccinated loved ones without fear...

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What Is Vaccine Shedding?

Vaccine shedding refers to the release of weakened virus particles from a person who has received a live-attenuated vaccine—a type of vaccine that uses a weakened form of a virus to stimulate immunity. These vaccines are designed to replicate, or make copies of themselves, in a limited and controlled way in the body to trigger a strong immune response. Sometimes the weakened virus can be shed (released) through bodily fluids like stool or respiratory droplets.

Examples of live-attenuated vaccines include:

• Measles, mumps, and rubella (MMR) vaccine

• Varicella (chickenpox) vaccine

• Oral polio vaccine (OPV) – no longer available in the U.S.

• Rotavirus vaccine

• Intranasal flu vaccine (FluMist)

Importantly, shedding does not mean transmission. When a weakened virus is shed, it is unlikely to cause illness in others. In fact, the only live vaccine historically associated with transmission is the oral polio vaccine (OPV), which is no longer used in the U.S.

OPV, which contains live attenuated (weakened) poliovirus, has been a game changer in the fight to eradicate (wipe out) polio worldwide. It’s given by mouth in liquid form, so it triggers strong protection in the gut, also called mucosal immunity, and is shed in the stool for several weeks. Weakened virus that is shed in the stool can pass to close contacts and boost immunity across the community. While the shedding helps to build protection, it can sometimes lead to problems. In rare cases, especially in areas where many people aren’t fully vaccinated, the weakened virus can change back into a harmful form, leading to outbreaks of vaccine-derived poliovirus (VDPV).

Shedding is usually safe when most people in an area are vaccinated; this highlights why it’s so important to keep vaccination rates high, which will allow us to eventually switch over to using the inactivated (killed) polio vaccine (IPV) and completely eradicate polio while preventing these risks.

What About COVID-19 Vaccines?

None of the COVID-19 vaccines authorized in the U.S. or globally are live attenuated vaccines. They use technologies like mRNA, viral vectors, or protein subunits—none of which contain live virus capable of replication or shedding. Claims that vaccinated individuals can “shed” spike proteins or other components and affect others are not supported by any scientific evidence.

Is Shedding Dangerous?

For the vast majority of people, vaccine shedding is not a health risk. Sometimes immunocompromised people may be advised by their healthcare provider to avoid close contact with someone recently vaccinated with a live attenuated vaccine as a precautionary measure. But for most people, being near someone who has been immunized with a live attenuated vaccine is not a concern.

Why This Myth Persists

The idea of vaccine shedding took on a life of its own during the COVID-19 pandemic. It’s a perfect example of how a kernel of scientific truth - that some live vaccines can shed weakened virus - gets stretched and distorted into something unrecognizable. Mix in genuine confusion about how different vaccines work, add a dash of fear about new technology, and you get widespread worry about something that doesn’t apply to most vaccines at all. The science tells a much less dramatic story: vaccine shedding only happens with certain live vaccines, it’s expected when it does occur, and it rarely causes problems.

Final Thoughts

Vaccine shedding is a real phenomenon that’s been completely taken out of context. Yes, some live vaccines can shed weakened virus. Yes, in very specific circumstances with severely immunocompromised people, we take extra precautions. But these kernels of truth have been twisted into a scary story that bears little resemblance to reality.

Most vaccines can’t shed at all. The ones that do? They’re shedding weakened virus that usually can’t even cause illness. And ironically, the people most worried about shedding are often missing the bigger picture - that vaccines are one of the most powerful tools we have to prevent disease. Infection from exposure to the unmodified virus—the one found in nature—is what we actually want to avoid. Live attenuated vaccines are safe and effective at preventing these diseases.

Understanding how vaccines work—including the rare phenomenon of shedding—helps us make informed decisions and separate fact from fiction.

If you have questions or concerns, talk to a trusted healthcare provider or consult reputable sources like the Mayo Clinic, the Children’s Hospital of Philadelphia (CHOP) Vaccine Education Center (VEC), or the American Academy of Pediatrics (AAP) Healthy Children website.

The next time someone brings up vaccine shedding, you can tell them that yes, it exists in very specific contexts. No, it’s not the danger they think it is. Sometimes the most boring explanation is the right one.

And me? I’ll stick to worrying about the shedding in my shower drain. At least that’s a problem I can actually see (and unfortunately, have to clean up).

Stay Curious,

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